The present invention relates to the crystal growth of a III-V compound semiconductor composed of at least one of what is called the group-III elements including B, Al, Ga, In and Tl and at least one of what is called the group-V elements including N, P, As, Sb and Bi, or more in particular to crystal growth techniques desirable for forming the crystal of a structure having the hexagonal symmetry structure or the crystal of a III-V compound (hereinafter referred to as xe2x80x9cthe nitride semiconductorxe2x80x9d) required to contain N (nitrogen) as a group-V element.
Also, the present invention relates to a semiconductor device composed of the crystal having a structure of the hexagonal system or a nitride-semiconductor, and to semiconductor light-emitting diodes and semiconductor laser devices suitable for emitting the light with wavelengths up to the ultra=violet ray or suitable as a light source for optical information processings or a light source for optical measurement equipments.
In recent years, various reports on the diodes and laser devices for emitting a light in the wavelength of blue region using GaInNIGaN/AlGa materials have been published in Appl. Phys. Lett., Vol. 64, March 1944, pp. 1687-1689 (Article 1); Appl. Phys. Lett., Vol. 67, September 1995, pp. 1868-1870 (Article 2); Jpn. J. Appl. Phys., Vol. 34-7A, July 1995, pp. L797-L799 (Article 3); Jpn. J. Appl. Phys., Vol. 34-10B, October 1995, pp. L1332-L1335 (Article 4); and Jpn. J. Appl. Phys., Vol. 34-11B, November 1995, pp. L1517-L1519 (Article 5).
What is shared by the semiconductor devices disclosed in Articles 1 to 5 is that a buffer layer composed of the above-mentioned nitride semiconductor is formed on a sapphire (Al2O3) substrate, and a nitride semiconductor layer is grown on the buffer layer. Such a structure is disclosed in JP-A-4-297023 (and JP-A-7-312350 constituting a divisional application thereof, and a corresponding U.S. application patent No. 5,290,393) and JP-A-4-321280. According to the teaching of JP-A-4-297023, a polycrystalline layer is produced by forming a first nitride semiconductor layer made of GaxAl1-xN (0xe2x89xa6xxe2x89xa61) on a sapphire layer at 300 to 700xc2x0 C. lower than the melting points of these materials. When a second nitride semiconductor layer is grown on this polycrystalline layer at a temperature of 1000 to 1050xc2x0 C., the second nitride semiconductor layer is epitaxially grown with the grains (crystal grains) constituting the first nitride semiconductor layer as nuclei. As a result, an epitaxial film of a nitride semiconductor having fine surface morphology can be formed on the sapphire substrate. A proposal thus has been made to utilize the above-mentioned polycrystalline layer as a buffer layer for forming a semiconductor device.
The reason why a semiconductor device composed of the above-mentioned nitride semiconductor is formed on a sapphire substrate is, as disclosed in JP-A-6-101587, that the crystal structure of sapphire, unlike that of GaAs or the like (having a cubic symmetry structure of zinc-blende type), has a hexagonal closed-packing structure (also called hexagonal zinc sulfide or wurtzite structure). According to this publication, however, the difference in lattice constant between GaN and sapphire is as large as about 14%. The nitride semiconductor layer formed on the sapphire substrate, therefore, develops lattice defects such as dislocations, so that the non-saturated bonding caused in the nitride semiconductor layer forms a doner level or absorbs elements of impurities constituting donors. The resulting problem is that this nitride semiconductor layer assumes N type and the life time of the carriers injected into this nitride semiconductor layer is shortened. This publication, in order to solve this problem, discloses a technique which employs a substrate made of MgAl2O4 having a cubic symmetry spinel crystal structure or MgO having a NaCl-type crystal structure and fabricates nitride semiconductor layers on the substrate by matching the lattice constants between them. A semiconductor laser using this technique is reported in Appl. Phys. Lett., Vol. 68, April 1996, pp. 2105-2107 (Article 6).
The above-mentioned conventional techniques teach the possibility of realizing a semiconductor device made of what is called a nitride semiconductor required to contain N (nitrogen) as a III-V chemical compound semiconductor or a group-V element having a crystal structure of the hexagonal symmetry structure. Nevertheless, sufficient data (for example, the continuously operating time of a laser device) are not available to guarantee the practicability of such a semiconductor device. Especially, the supplementary trials conducted by the inventors show that the density of defects developed in the nitride semiconductor layer is as high as 1011 cmxe2x88x922, and the inventors judged that a laser device capable of being operated continuously for at least 1000 hours cannot be realized under the above-mentioned conditions.
In recent years, NIKKEI ELECTRONICS, Dec. 4, 1995 issue, No. 650, pp. 7 (Article 7) has reported that as a result of a joint research made between Cree Research, Inc. and North America Phillips, it was found that the use of SiC crystal as a substrate reduces the lattice defect density of the nitride semiconductor layer formed on the substrate to as low as 108 cmxe2x88x922 and thus can realize blue laser diodes higher in brightness than the conventional devices. According to Article 7, however, the defect density of the nitride semiconductor layer is insufficient to lengthen the life time of the laser diode, and the reduction in the defect density (104 cmxe2x88x922 at present) of the crystal of the SiC substrate is indispensable for reducing the defect density of the nitride semiconductor layer. In constructing a semiconductor laser by forming a nitride semiconductor layer on a SiC substrate, therefore, an improved quality of the SiC substrate as well as the growth of a nitride semiconductor layer is indispensable for lengthening the life time, and the development cost is expected to increase.
Also, the articles and publications introduced above refer to the configuration of a nitride material used for an optical active layer or an optical waveguide layer but not to the shape of the active layer or the waveguide for controlling the transverse mode of the semiconductor laser. Thus, none of the articles and publications contain the description of a method of reducing the crystal defect density suitable for the waveguide structure mentioned above or, especially, a method of reducing the optical loss in the neighborhood of the active layer of the waveguide.
A first object of the present invention is to realize a crystal growth technique for forming a semiconductor layer having a very low defect density made of a III-V compound semiconductor having a crystal structure of the hexagonal symmetry structure or made of what is called a nitride semiconductor required to contain N (nitrogen) as a group-V element. xe2x80x9cA very low defect densityxe2x80x9d indicates a defect density on the order of 107-cmxe2x88x922 or less which is difficult to attain by the above-mentioned technique using a SiC substrate. This crystal growth technique not only includes a technique for reducing the defect density of a fabricated semiconductor layer uniformly as a whole but also is aimed at examining what is called the selective crystal growth for reducing the defect density only in the desired region.
A second object of the present invention is to lengthen the life time of the semiconductor device fabricated using the above-mentioned crystal growth technique or to improve the life or mobility of the carriers involved in the operation of the semiconductor device to a sufficient value for practical application of the semiconductor device. Especially with the light-emitting diode or the laser diode, the second object of the invention is to define a configuration of an optical waveguide suitable for geometrically controlling or reducing the optical loss of the waveguide and the active layer based on the above-mentioned selective crystal growth method, and a configuration suitable for obtaining a stimulated emission light with high internal quantum efficiency from a flat or smooth active layer of the laser device. Specifically, the technique according to one aspect of the second object of the invention is aimed at defining the structure of a waveguide and an active layer capable of guiding the fundamental transverse mode stably in the wavelength of blue-violet region and suitable for realizing a laser diode operating with low threshold current and high efficiency.
1. Introduction
One of the present inventors proposed a semiconductor laser device as described below in the specification of JP-A-7-238142 forming the foundation for the declaration of priority of the present application. An example will be described with reference to FIG. 1. A first nitride semiconductor layer (including a GaN buffer layer 2 and a n-type GaN optical waveguide layer 3) is formed by crystal growth on a single-crystal substrate 1 of sapphire (xcex1-Al2O3) having a (0001)C surface, and then an insulator mask 4 is formed on the first nitride semiconductor layer. This insulator mask 4 has regular patterns of rectangular window regions, in which the upper surface (the upper surface of the n-type GaN optical waveguide layer 3) is exposed. Under this condition, a second nitride semiconductor layer (a n-type GaN optical waveguide layer 5) is selectively grown on the insulator 4 (on the first nitride semiconductor layer in the window regions). FIG. 1B is a plan view of the insulator mask 4, and FIG. 1A is a sectional view taken in line A-Axe2x80x2 in FIG. 1B.
Specifically, the semiconductor laser device proposed by one of the present inventors has a feature in that a nucleation region for crystal growth of the second nitride semiconductor layer is confined to the surface of the n-type GaN optical waveguide layer 3 exposed in the above-mentioned window regions, and by thus improving the three-dimensional growth density, the second nitride semiconductor layer is grown in such a manner as to fill the window regions first and when the uppermost surface of the grown portions reaches a level flush with the upper surface of the insulator 4, the growth of the second nitride semiconductor layer is started on the insulator 4. The surface of each portion of the second nitride semiconductor layer protruded from each window region grows to extend in the directions parallel and perpendicular to the upper surface of the insulator 4. In the case where the window areas of substantially the same size are arranged regularly (or substantially equidistantly), therefore, the crystal layers that have extended from each pair of adjacent window regions coalesce with each other on the insulator 4 substantially at the same time. This produces the effect of reducing the crystal defects or crystal grain boundaries of the second nitride semiconductor layer, as compared with the normal bulk growth in which the second nitride semiconductor layer is formed on the insulator 4 as a crystal layer having a flat surface of growth without using any insulator mask.
The present inventors fabricated several lots of the above-described semiconductor laser devices according to the present invention on an insulator of SiO2, and by reducing the thickness of these lots along the direction of crystal growth, observed them under the transmission electron microscope (TEM). The semiconductor laser devices each was fabricated by forming a first nitride semiconductor layer, an insulator having openings (window regions) and a second nitride semiconductor layer, in that order, on the (0001)C surface of a sapphire (xcex1-Al2O3) single-crystal substrate. Also, a portion of the second nitride semiconductor layer is formed in the openings, and coupled to the first nitride semiconductor layer on the bottom of the openings. The following knowledge has been obtained from what is called the cross-sectional TEM image of this semiconductor laser device.
Knowledge 1: The crystal defect density of the second nitride semiconductor layer grown on the SiO2 film (insulator) is in or lower than the range of 104 to 105 cmxe2x88x922. In contrast, the crystal defect density of the second nitride semiconductor layer grown from the upper surface of the first nitride semiconductor layer in the openings of SiO2 is at the same level of 109 to 1011 cmxe2x88x922 as reported in the past. Most of the defects (dislocations) observed in the second nitride semiconductor layer grown in the openings are originated in the interface of the sapphire substrate and intrude into the openings through the first nitride semiconductor layer, while only a small portion of the defects intrudes into the second nitride semiconductor layer formed on the SiO2 layer. In other words, the defects observed in the second nitride semiconductor layer on SiO2 sharply decrease away from the SiO2 openings. Origination of the defects is shown schematically by dashed lines in FIG. 1.
Applying Knowledge 1 to the unit structure of the hexagonal crystal system shown in FIG. 2, the present inventors confined the nucleation for crystal growth of a nitride semiconductor crystal by forming openings in an insulator, and with regard to a nitride semiconductor laser device formed with an increased nucleation density, the inventors have examined as follows:
The growth of the crystal having a hexagonal zinc sulfide (wurtzite) structure which is a kind of a hexagonal crystal system has such a feature that defects extend selectively along c-axis but do not multiply along other axes. A transmission electron diffraction pattern has confirmed that the second nitride semiconductor layers grown in the openings and on the insulator are both the crystal of wurtzite structure. The nitride semiconductor layer on the insulator, however, is substantially free of the defects extending along c-axis constituting the feature of the wurtzite structure. Suppose that the second nitride semiconductor layer is divided into two regions about imaginary interfaces (indicated by one-dot chains in FIGS. 1A and 1B) extending along the side walls of the openings. Almost all the defects in the regions on the insulator are successors to the defects formed in the openings and are received at the imaginary interfaces. Consequently, on the assumption that the region on the insulator grows in the direction substantially perpendicular to c-axis from the imaginary interfaces (i.e., in what is called homoepitaxial growth), the low defect density in the particular region is considered due to the property of crystal growth of the wurtzite structure in which no defect multiplies along other than c-axis direction.
In view of this, the present inventors have concluded as follows:
Conclusion 1: The crystal of a nitride semiconductor grows extending in vertical direction from the surface of the region having a crystal structure in the openings of the insulator (i.e., in the direction perpendicular to the particular surface) following the atomic arrangement of the same surface, and the nitride semiconductor grown and protruded out of the openings extends onto the insulator in transversal direction (i.e., in the direction substantially parallel to the upper surface of the insulator) from the sides of the nitride semiconductor protruded from the openings as new growth interfaces. In other words, the crystal growth of a nitride semiconductor is what is called the selective crystal growth exhibiting a behavior in the openings different from that on the surface of the insulator. In the latter case, i.e., on the surface of the insulator, the crystal growth is substantially homoepitaxial.
The c-axis is an axis of the coordinates (called the crystal axis) for defining the atomic arrangement of the unit cells of the crystal of the hexagonal symmetry structure. In FIG. 2, the c-axis is designated by arrow (unit vector) c (a1 axis, a2 axis and a3 axis are also similarly designated). In FIG. 2, group-III atoms (Ga, Al, etc.) are designated by white circles, and group-V atoms (N, As, etc.) by black circles. The crystal surface of the hexagonal symmetry structure is expressed by index (a1, a2, a3, c) defined by these unit vectors, an example of which is shown in FIG. 2. Sapphire has also a crystal structure of the hexagonal system like a nitride semiconductor, and therefore the (0001)C surface thereof is a crystal surface orthogonal to c-axis as obvious from FIG. 2. The a1-axis and a2-axis in the crystal structure of the hexagonal symmetry structure are alternatively expressed as the a-axis and the b-axis, respectively. On the basis of this notation, the a-axis, b-axis and the c-axis are sometimes expressed by the index [a, b, c] which normally represents [100], [010] and [001]. The fundamentals of the crystal structure of the hexagonal system were described above. With reference to FIG. 2, it will be understood that the above-mentioned first nitride semiconductor layer epitaxially grows along c-axis on the (0001) plane of the sapphire crystal substrate which is also the hexagonal symmetry structure. (The crystal of the first nitride semiconductor layer grown in this way is called to be xe2x80x9coriented along c-axisxe2x80x9d with respect to the sapphire substrate).
For in-depth examination of the truth of the selective growth of the nitride semiconductor based on Conclusion 1, the present inventors have examined the crystalline property of the crystal structure (amorphous or single crystal) and the component elements of the insulator having the openings and the crystalline property of the nitride semiconductor formed in a manner to cover the insulator. In conducting this examination, as shown in FIGS. 3A to 3E, an insulator 4 having an opening 40 is formed or bonded on the (0001)C surface of a sapphire substrate 1, and like in the semiconductor laser device described above, a nitride semiconductor crystal 5 was grown on the insulator while confining the nucleation region to the opening. As the result of varying compositions of the insulator and the nitride semiconductor by lots, the following knowledge were obtained:
Knowledge 2: In the case where the insulator is made of an amorphous material such as SiO2, Si3N4 (SiNx), SiO2:P2O5 (PSG), SiON or Ta2O5, the defects in the nitride semiconductor layer formed on the insulator sharply decrease with the distance away from the opening. Also, the crystal defect density in the nitride semiconductor layer formed on the insulator is in or lower than the range of 104 to 105 cm2.
Knowledge 3: In the case where the insulator is made of SiC or BaTiO3 having a crystal structure, on the other hand, the growth of a nitride semiconductor started in the opening (on the sapphire substrate) at the same time as on the insulator. The grown surfaces of the nitride semiconductors had unevenesses reflecting the presence or absence of a window region, and crystal defects developed on the insulator in a density (in the range of 108 to 1011 cmxe2x88x922) comparable to that in the opening.
Knowledge 4: Further, in the case where the surface of the sapphire was non-crystallized partially by irradiating Ga ions and a nitride semiconductor layer was grown on the non-crystallized surface, crystal has begun to grow on the non-crystallized surface almost at the same time as on the surface maintaining a crystal structure. It was found that the crystal defect density of the nitride semiconductor layer formed on the non-crystallized surface is lower than that on the surface having a crystal structure, but inferior (in the range of 106 to 108 cmxe2x88x922) to the crystal defect density obtained in the nitride semiconductor layer formed on the SiO2 layer.
In each of the foregoing experiments, the density defect in the nitride semiconductor layer grown in the opening was in the range of 108 to 1011 cmxe2x88x922. These experiments will be described in more detail later with reference to embodiments of the invention.
The present inventors reached the following conclusions from Knowledge 2 to 4:
Conclusion 2: In the case where the insulator forming the base of a nitride semiconductor is amorphous, the growth of the crystal of the nitride semiconductor becomes more active in the direction perpendicular to what is called the c-axis along which defects are not liable to extend easily. In the case where the insulator has a crystal structure, on the other hand, the crystal of the nitride semiconductor grows heteroepitaxially as defined by the atomic arrangement in the surface of the insulator. In other words, for the defects of a nitride semiconductor crystal to be reduced, it is indispensable to remove the effects that the atomic arrangement of an insulator may have on the atomic arrangement in the surface of the particular insulator, and therefore the insulator is required to have an amorphous structure.
Further, in order to verify Conclusion 2, the present inventors attached a minuscule droplet of Ga atoms to the center of the amorphous surface of an insulator formed uniformly and substantially flatly, and heated the resulting assembly to the growth temperature of 1030xc2x0 C. in the ammonia environment. This experiment will be described in more detail later in the section of xe2x80x9cIntroductionxe2x80x9d to embodiments of the invention. For the time being, only the result of the experiment will be described. This experiment, in order to clarify the mechanism of crystal growth on an insulator, oxidized the surface of a silicon single-crystal substrate and formed an amorphous SiO2 film 101. A nitride semiconductor was grown on this SiO2 film 101 without forming any openings therein. The following knowledge was obtained from this experiment:
Knowledge 5: A microcrystal having a mirror surface was formed in the region formed with the droplet of Ga atoms. Further, in the case where the temperature of the amorphous SiO3 film was held at the growth temperature of 1030xc2x0 C. and a trimethyl gallium (TMG) gas was supplied in the ammonia environment, a single crystal in the shape of hexagonal column was gradually grown about the above-mentioned microcrystal. An observation of the section of this single crystal under TEM shows that the density of the defects found in the crystal is considerably lower than the value (104 to 105 cmxe2x88x922) for the above-mentioned selective growth. In fact, some lots were regarded substantially free of defects.
On the basis of the above-mentioned experiment and Knowledge 5 obtained therefrom, the present inventors interpreted the crystal growth mechanism for the nitride semiconductor on an insulating material according to the model shown in FIGS. 4A to 4E. In the model of FIGS. 4A to 4E, Ga atoms (group-III elements) are designated by white circles, and N atoms (group-V elements) are designated by black circles. Also, the direction of movement of each atom was indicated by an arrow attached to each circle. Opinions of the present inventors will be described in detail below.
According to the interpretation of the present inventors, the N atoms and the Ga atoms on the amorphous insulator move about actively in search of a stable state. This is similar to the behavior of Si atoms forming terraces and atomic steps in the topmost surface of silicon, known as the atomic migration. In the experiment under consideration, Ga atoms were fixed as a droplet (FIG. 4A) on an amorphous insulator. Exposure to an ammonia environment under this condition causes the N atoms supplied from the environment to attach on the insulator and reach the droplet of Ga atoms. As described above, N atoms of the group-V element, which share the electrons in the outermost cell with Ga atoms of the group-III element, form a pair in a stoichiometric ratio of Ga:N=1:1. In this way, a stability is attained by forming a compound (FIG. 4B).
Further, the insulator is set at an optimum temperature for growing a single crystal of GaN. Then, the N atoms concentrated in the Ga droplet build up a mutually regular arrangement in order to improve the stability of the compound, thereby forming the above-mentioned microcrystal (FIG. 4C). Under this condition, extraneous N atoms not participating in the formation of a GaN crystal exist on the insulator. Supplying TMG in this state, the ratio between the number of Ga atoms and the number of N atoms existing on the insulator approaches the above-mentioned stoichiometric ratio. In view of the fact that both Ga atoms and N atoms move about rapidly on the insulator, however, the probability of the two types of atoms bumping each other is negligibly low as compared with the probability of their bumping the GaN crystal fixed on the insulator. Thus, all of the N atoms and Ga atoms supplied from the TMG and the ammonia environment substantially participate in the growth of the GaN crystal (FIG. 4D). As a result, the crystal growth proceeds over the entire crystal surface of the Ga microcrystal, so that the GaN single crystal multiplies while maintaining the shape of a hexagonal column (FIG. 4E). As obvious from the above-mentioned process, the direct growth of a nitride semiconductor crystal on an insulator starts with the Ga atoms supplied as a droplet on the insulator as a nucleus.
From the foregoing interpretation, the present inventors have obtained the following conclusion:
Conclusion 3: A nitride semiconductor layer can be grown directly on an insulator of an amorphous structure without depending on the selective crystal growth described above. In this case, it is necessary to form a nucleus for crystal growth on the insulator. This nucleus is sufficiently composed of atoms of a group-III element alone.
Conclusion 3 indicates that no new crystal growth occurs unless a nucleus for crystal growth is supplied on the insulating layer on the one hand and that an unexpected crystal growth occurs depending on the type of atoms existing in the surface of an insulator having an amorphous structure on the other. Specifically, according to Conclusion 3, it is possible to explain consistently, as described below, that a nitride semiconductor layer begins to grow on the non-crystallized surface of a sapphire substrate obtained as Knowledge 4 almost at the same time as on the surface maintaining a crystal structure, and that the crystal defect density of such a nitride semiconductor layer is higher than that of the nitride semiconductor layer grown on a SiO3 film.
First, the substantially simultaneous crystal growth in the non-crystallized portion and the crystal portion is caused by the fact that the Al atoms constituting a component element of the sapphire existing in the non-crystallized surface form a nucleus of a nitride semiconductor layer as a group-III element. In other words, the Al atoms play the same role as the droplet of Ga atoms described in Knowledge 5. Consequently, micro-crystals are formed in an irregular arrangement on the non-crystallized surface of the sapphire constituting an insulator of an amorphous structure, and the individual single-crystal regions grown therefrom coalesce with each other discretely during the crystal growth time, thereby giving rise to an unexpected mutual stress between the single-crystal regions. Especially, slight ups-and-downs of the surface of the insulator presents itself as a difference in the orientation angle of c-axis between the single crystal regions. Thus, the crystal in one region grows in such a direction as to bite into the crystal in another region, with the result that a stress which induces crystal defects is generated between the regions involved. The nitride semiconductor layer on the non-crystallized portion thus develops a great number of crystal defects, though not as much as experienced by the nitride semiconductor layer on the crystal portion (i.e, the portion affected by lattice mismatching).
The above-mentioned experiment revealed the fact that in growing a nitride semiconductor layer directly on an amorphous insulator, it is critical to control the distribution of the group-III elements on the surface of the insulator, and it became apparent that the insulator is desirably formed of a material not containing any group-III element as a constituent element. In other words, the composition of the material constituting the amorphous insulator is preferably different from that of the semiconductor layer formed on the amorphous insulator. Further, it is desirable not to contain any group-III element as a constituent element. Although the foregoing examination concerns a nitride semiconductor, i.e., what is called a III-V compound semiconductor which is composed of at least one of the group-III elements including B, Al, Ga, In and Tl on the one hand and at least one of the group-V elements including N, P, As, Sb and Bi on the other hand and which contains N (nitrogen) as a group-V element, the inventors have concluded that the same result of examination can be fed back for use with a semiconductor crystal constituting what is called a III-V compound having a structure of the hexagonal symmetry structure.
Based on the above-mentioned result of examination, the present inventors propose below a semiconductor material having a new configuration and a method of fabrication thereof. The semiconductor material referred to herein is not limited to those employed in the structure of a semiconductor device but includes, for example, a body on which a semiconductor device is formed.
Semiconductor material 1: This material comprises a first region made of a crystal of a compound semiconductor containing at least nitrogen as a constituent element and a second region made of an insulator, wherein at least a portion of the first region is grown on the second region.
Semiconductor material 2: This material comprises a first region made of a compound semiconductor having a crystal structure of the hexagonal symmetry structure and a second region made of an insulator having an amorphous structure, wherein at least a portion of the first region is grown on the second region.
The above-mentioned semiconductor materials include those characterized in that the density of defects existing in the crystal of the portion of the first region grown on the second region is not more than 107 cmxe2x88x922. The above-mentioned semiconductor materials also include those characterized in that the compound semiconductor making up the first region is composed of a group-III element and a group-V element.
Semiconductor material fabrication method 1: This method is characterized by comprising a step of growing the crystal of a compound semiconductor containing at least nitrogen as a constituent element on the surface of an insulator having an amorphous structure.
Semiconductor material fabrication method 2: This method is characterized by comprising a step of growing the crystal structure of a hexagonal symmetry structure of a compound semiconductor configured of a group-III element and a group-V element on the surface of an insulator having an amorphous structure.
The above-mentioned methods of fabricating a semiconductor material, in which an insulator is formed on a crystal substrate of the hexagonal system and has at least an opening.
The above-mentioned semiconductor materials and methods for fabrication thereof are promising as a technique for providing a nitride semiconductor now closely watched as a material of a light-emitting diode for emitting the light of wavelengths from green to ultra-violet ray, i.e. a material with low defect density having a crystal structure of the hexagonal system composed of at least one of the group-III elements (especially, Ga, Al and In) and the N (nitrogen) element.
2. Application to Semiconductor Devices
The inventors propose a configuration of a semiconductor device realized by the above-mentioned fabrication technique of a semiconductor material. Specifically, the present invention provides a semiconductor device fabricated by forming a conventional nitride semiconductor by heteroepitaxial growth, in which the structural problem of the conventional semiconductor device unavoidably caused by the heteroepitaxial growth is avoided by combining the above-mentioned technique of transverse homoepitaxial growth.
The present invention will be briefly explained with reference to an application to a semiconductor optical device (a general term for a semiconductor laser device, an optical modulator and an optical switch) as an example of a semiconductor device. The significant basic feature of the semiconductor optical device according to this invention is that a semiconductor layer constituting an optical crystal region (a general term indicating regions for emitting, absorbing, containing or guiding the light, including an active layer and an optical waveguide) is formed on an amorphous insulator. Specifically, a semiconductor layer making up an optical crystal region or a semiconductor layer forming the base thereof is formed by the transverse homoepitaxial growth technique described above thereby to reduce the density of the defects generated in the crystal layer. From the viewpoint of the process for fabricating the device, unlike in the prior art for forming an optical crystal region by repeating the heteroepitaxial growth on the main surface of a substrate, the invention is characterized in that an insulator having at least an opening is formed on the main surface of a substrate and a semiconductor layer is formed by homoepitaxial growth on the insulator, after which an optical crystal region is formed as a semiconductor layer in or on the homoepitaxial layer by heteroepitaxial growth.
As described above, in the transverse crystal growth of a nitride semiconductor occurring on an region composed of an insulator having an amorphous structure (such as a body of an insulator), the semiconductor crystals grown transversely from different regions are transversely coalesced on the insulator region or on the body. This insulator region is formed as a mask-like insulator having at least an opening, and thus the growth of the crystal of a nitride semiconductor layer on the insulator is controlled, thereby considerably reducing the crystal defects such as dislocations in the nitride semiconductor layer formed on the insulator. With the semiconductor optical device according to the present invention characterized in that an optical crystal region is formed in a nitride semiconductor layer formed on the insulator or in a semiconductor layer formed by epitaxial growth on the nitride semiconductor layer, the crystal defect density in the optical region can be suppressed within or lower than the range of 104 to 105 cmxe2x88x922. With the conventional semiconductor optical device with an optical crystal region formed by sequential heteroepitaxial growth of the crystal of a nitride semiconductor on a crystal substrate (such as a sapphire substrate) having a different lattice constant, on the other hand, the crystal defect density occurring in the optical crystal region is 108 to 1011 cmxe2x88x922.
The reduction of the crystal defects in the optical crystal region according to the present invention obviates all the problems of the scattering loss of light and the shortened life of the carriers contributing to light emission due to crystal defects at a time. Especially, the reduced crystal defects of an optical waveguide suppresses the loss of optical gain due to the scattering in the resonative amplification of stimulated emission light and therefore secures an operation with low threshold current and high efficiency.
Also, if a semiconductor layer between an optical crystal region and an electrode (hereinafter referred to as the contact layer) is formed by homoepitaxial growth on an insulator, an increased amount of n-type or p-type impurities can be doped into the contact layer. As a result, carriers can be easily generated by doping impurities into the contact layer, thereby making it possible to set the concentration of n- and p-type carriers to a high value on the order of 1018 to 1019 cmxe2x88x923. Consequently, in an application to a semiconductor laser device, for example, carriers of high density can be injected toward the optical active layer from the optical waveguide layer due to the reduced resistance of the contact layer. Thus, the optical gain can be improved and hence an operation can be performed with low threshold current and high efficiency.
The use of the waveguide structure described below is effective for improving the effect of a semiconductor optical device according to the invention. Well-known waveguide structures include a gain-guided structure with a limited region of an optical active layer for securing a gain by a current-blocking layer, and a refractive index-guided structure having a refractive index difference transversely of an optical active layer by such a stripe structure as a ridge stripe structure or a buried heterostructure (BH). Among these structures, the refractive index-guided structure, which can guide the wave stably in fundamental transverse mode, is especially important for assuring an operation with low threshold current and high efficiency. According to this invention, a substantial optical crystal region defined by the waveguide (i.e., the portion at which carriers are injected or where an electric field is applied) is set on an insulator so that carriers are injected and the electric field is applied intensively into an optical active layer and an optical waveguide layer composed of a semiconductor crystal of low defect density, thereby improving the emission efficiency with respect to the amount of injected carriers and the optical modulation efficiency according to the electric field intensity. The waveguide structure described above can be formed on, under or in the optical crystal region (in an optical guide layer or in a cladding layer). As a result, a refractive index-guided structure for propagating the laser beam stably in fundamental transverse mode can be realized while at the same time forming a waveguide structure of a crystal layer low in defect density and high in quality. A semiconductor laser operating with low threshold current and high efficiency can thus be produced. Also, a semiconductor laser device in a stripe extended in the direction of laser cavity is recommended, the shape of which is not of course limited. Further, a striped current-blocking layer and a layer for forming a refractive index difference, which can be formed by etching a semiconductor layer, can alternatively be formed newly as an insulator having at least an opening. In such a case, the refractive index-guided structure can be easily realized by selectively growing a semiconductor layer using an insulator having a stripe pattern as a mask.
Furthermore, in order to improve the crystallinity of the semiconductor layer constituting an optical crystal region, it is recommended that dummies be formed by selective crystal growth in such positions as to sandwich the optical crystal region. For the purpose of configuring a waveguide layer composed of a crystal layer of higher quality, for example, an insulator is used which has a dummy pattern on each side of a pattern (opening) for producing a waveguide structure by selective growth. By doing so, the unusual growth is avoided in forming a waveguide at the central portion and thus the crystallinity and the geometric controllability of the particular waveguide are remarkably improved. Also, the threshold current and the operating efficiency of the device are further improved by a configuration in which current is prevented from being injected from the electrode formed on the waveguide into the crystal layers grown in the dummy pattern.
The single-crystal substrate used for fabricating a semiconductor optical device according to the present invention is not limited to the above-mentioned sapphire substrate. The points to be noted in using a new single-crystal substrate in place of the sapphire substrate are described below. In the case of using a single-crystal substrate of hexagonal wurtzite structure, for example, the orientation of the substrate surface is set to (0001)C plane. When fabricating a stripe structure on this substrate, the insulator mask pattern is set in the direction perpendicular or parallel to the (1120)A plane of the substrate. In this way, individual waveguide crystal layers having a rectangular section can be coalesced, thereby making it possible to fabricate a single large waveguide structure easily by use of the selective growth technique. In the case of using a single-crystal substrate of a cubic zinc-blende structure, on the other hand, the orientation of the substrate surface is set to the (111) plane. When fabricating a stripe structure on this substrate, an insulator mask pattern is set in the direction perpendicular to the (110) plane or perpendicular to the (1-10) plane of the substrate. A waveguide structure having a crystal in the shape similar to the above-mentioned case can thus be fabricated.
Various types of semiconductor optical devices according to the invention can be realized by employing various shapes of the openings formed in the insulator constituting the base for homoepitaxial growth. First, by quantizing the width of the openings of the insulator one-dimensionally or two-dimensionally transversely of an active layer, an optical active layer is produced with a quantum box structure or quantum wire advantageous for the operation of the laser device with low threshold current. Also, if a plurality of parallel striped openings are formed and the phase-matching conditions are appropriately regulated for the light generated from each semiconductor layer (active layer) selectively grown between the stripes, then a semiconductor device of a phased array structure can be configured thereby to achieve a high-output operation in fundamental transverse mode.
A semiconductor device according to the invention was described above taking a semiconductor optical device as an example. Nevertheless, the optical crystal region can be replaced with a region in which switchable carriers flow, thereby constituting a field effect transistor, for example. In the case of a field effect transistor, a semiconductor layer making up what is called a channel for activating the carriers is desirably formed on an insulator. An even higher effect is produced if the source region, the gate region, the drain region, etc. for this channel are formed on the insulator. The feature of an example of a preferable device configuration is that a source electrode, a gate electrode and a drain electrode are formed in juxtaposition on a uniform (openingless) amorphous and insulative region through a semiconductor region constituting a channel. This semiconductor device configuration is employed especially effectively for a high electron mobility transistor (HEMT) easily affected by the crystal defect density of the channel region.
Based on the above-mentioned result of examination, the present inventors propose below a semiconductor device having a new configuration. The term xe2x80x9csemiconductor regionxe2x80x9d herein, unless otherwise specified, refers to the one formed of a nitride semiconductor material or a compound semiconductor material having a hexagonal crystal structure. Both semiconductor materials were defined in detail in xe2x80x9cIntroductionxe2x80x9d. Also, the xe2x80x9cinsulative regionxe2x80x9d is defined as a body or a layer (film) made of a material having an amorphous structure and exhibiting an electrical insulation property, and unless otherwise specified, is assumed not to be formed with any opening (i.e. a region where another region having a crystal structure is exposed). Any material meeting these conditions can be used to form an insulative region.
Semiconductor device 1: This semiconductor device is formed with a semiconductor region making up an optical system on an insulative region. The optical system represents the above-mentioned optical crystal region. With a laser device, a cavity structure for lasing is desirably arranged on the insulative region. This semiconductor device configuration can be employed for all of what are called semiconductor optical devices including a light-emitting diode (LED), a light transmission path and an optical modulator as well as for a semiconductor laser device.
Semiconductor device 2: This semiconductor device is formed with a semiconductor region composed of semiconductor layers having different band gaps (energy gaps) on an insulative region. These semiconductor layers constituting the semiconductor region include a first semiconductor layer and second semiconductor layers formed on and under the first semiconductor layer and having a larger band gap than the first semiconductor layer. The first semiconductor layer is used for injecting, confining or generating carriers. The second semiconductor layers, on the other hand, assist the first semiconductor layer in injecting or confining carriers. This semiconductor region can be what is called a quantum well structure with the thickness of the first semiconductor layer not more than the de Broglie wavelength or a multiple quantum well structure with the first and second semiconductor layers alternately formed in multiple stages. This device configuration is applicable also to a field effect transistor, a switching device and a logically operating device as well as to a semiconductor optical device. Also, the second semiconductor layers on and under the first semiconductor layer can have different compositions or different band gaps. This configuration is effective for realizing a field effect transistor, in which case the second semiconductor layer far from the gate electrode can be done without.
When any one of the above-mentioned semiconductor devices is fabricated in an existing semiconductor equipment, the above-mentioned insulative region can be formed on a region having a crystal structure, i.e., on a crystal substrate or a crystal layer (film).